1. Field of the Invention
The present invention relates to an optical apparatus, and more specifically, to an optical apparatus comprising an imaging apparatus having an image pickup device, a lens apparatus interchangeably attached to an imaging apparatus, and the imaging apparatus to which the lens apparatus is attached.
2. Description of the Related Art
A zoom lens system conventionally used for video cameras will be described.
Examples of zoom lens systems for video cameras include a four-component lens system comprising from the subject side a stationary positive lens component, a movable negative lens component, a stationary positive lens component and a movable positive lens component.
FIGS. 8(A) and 8(B) show the lens barrel structure of a general four-component zoom lens system. FIG. 8(B) shows a cross section taken on the line A—A of FIG. 8(A).
The four lens components 201a to 201d constituting this zoom lens system are a stationary front lens 201a, a variator lens component 201b moving along the optical axis to thereby perform zooming, a stationary afocal lens 201c, a focusing lens component 201d moving along the optical axis to thereby maintain the focal plane during zooming and perform focusing.
Guide bars 203, 204a and 204b which are disposed parallel to an optical axis 205 guide the movable lens components and prevent the movable lens components from rotating. A DC motor 206 serves as the driving source for moving the variator lens component 201b. 
The front lens 201a is held by a front lens barrel 202. The variator lens component 201b is held by a variator moving ring 211. The afocal lens 201c is held by an intermediate frame 215. The focusing lens component 201d is held by a focus moving ring 214.
The front lens barrel 202 is fixed to a rear barrel 216 so as to be positioned. By the barrels 202 and 216, the guide bar 203 is supported so as to be positioned, and a guide screw shaft 208 is supported so as to be rotatable. The guide screw shaft 208 is rotated by the rotation of an output shaft 206a of the DC motor 206 being transmitted through a gear train 207.
The variator moving ring 211 holding the variator lens component 201b has a presser bar spring 209, and a ball 210 engaging with a screw groove 208a formed on the guide screw shaft 208 by the force of the presser bar spring 209. By the guide screw shaft 208 being rotated by the DC motor 206, the variator moving ring 211 moves backward and forward in the direction of the optical axis while guided and rotation-restricted by the guide bar 203.
The guide bars 204a and 204b are supported by being engaged with the rear barrel 216 and the intermediate frame 215 positioned by the rear barrel 216. The focus moving ring 214 is movable backward and forward in the direction of the optical axis while guided and rotation-restricted by the guide bars 204a and 204b. 
A stop unit 235 (stop driving source 224) is fixed to the intermediate frame 215.
The focus moving ring 214 holding the focusing lens component 201d has a sleeve slidably engaged with the guide bars 204a and 204b. A rack 213 is attached so as to be integral with the focus moving ring 214 with respect to the direction of the optical axis.
A stepping motor 212 rotates a lead screw 212a integrally formed on the output shaft thereof. The rack 213 attached to the focus moving ring 214 is engaged with the lead screw 212a, and by the lead screw 212a rotating, the focus moving ring 214 moves in the direction of the optical axis while guided by the guide bars 204a and 204b. 
As the driving source of the variator lens component, a stepping motor may be used like the driving source of the focusing lens component.
A lens barrel body in which the lens components and the like are substantially enclosed is constituted by the front lens barrel 202, the intermediate frame 215 and the rear barrel 216.
When a lens component holding frame is moved by use of a stepping rotor as described above, it is detected that the holding frame is situated at one reference position in the direction of the optical axis by use of a photo interrupter or the like, and then, the absolute position of the holding frame is detected by continuously counting the number of driving pulses supplied to the stepping motor.
Next, the electric structure of a conventional imaging apparatus will be described with reference to FIG. 9. In this figure, the members of the lens barrel described with reference to FIGS. 8(A) and 8(B) are designated by the same reference numerals as those of FIGS. 8(A) and 8(B).
Reference numeral 221 shows a solid state image pickup device such as a CCD. Reference numeral 222 shows a driving source of the variator lens component 201b including the motor 206 (or a stepping motor), the gear train 207 and the guide screw shaft 208.
Reference numeral 223 shows the driving source of the focusing lens component 201d including the stepping motor 212, the lead screw shaft 212a and the rack 213.
Reference numeral 224 shows the driving source of the diaphragm unit 235 disposed between the variator lens component 201b and the afocal lens 201c. 
Reference numeral 225 shows a zoom encoder. Reference numeral 227 shows a focus encoder. The encoders 225 and 227 detect the absolute positions, in the direction of the optical axis, of the variator lens component 201b and the focusing lens component 201d, respectively. When a DC motor is used as the variator driving source as shown in FIGS. 8(A) and 8(B), an absolute position encoder such as a volume or a magnetic encoder is used.
When a stepping motor is used as the driving source, it is common practice to situate the holding frame at a reference position as mentioned above and then, continuously count the number of operation pulses input to the stepping motor.
Reference numeral 226 shows a stop encoder. As the stop encoder 226, for example, a type is used in which a Hall element is disposed in the stop driving source 224 such as a motor and the relationship between the rotation positions of the rotor and the stator is detected.
Reference numeral 232 shows a CPU controlling the camera. Reference numeral 228 shows a camera signal processing circuit performing predetermined amplification and gamma correction on the output of the solid-state image pickup device 221. The contrast signal of the image signal having undergone these predetermined processings passes through an AE gate 229 and an AF gate 230. That is, of she entire image plane, a signal extraction range optimum for deciding exposure and focusing is set by the gates. There are cases where these gates are variable in size and where a plurality of gates are provided.
Reference numeral 231 shows an AF (autofocus) signal processing circuit processing an AF signal for AF. The AF signal processing circuit 231 generates one output or more associated with the high-frequency component of the image signal. Reference numeral 233 shows a zoom switch. Reference numeral 234 shows a zoom tracking memory. In the zoom tracking memory 234, information of the focusing lens position to be set according to the subject distance and the variator lens position in zooming is stored. A memory in the CPU 232 may be used as the zoom tracking memory.
For example, when the zoom switch 233 is operated by the user, in order that the predetermined positional relationship between she variator lens and the focusing lens calculated based on the information in the zoom tracking memory 234 is maintained, the CPU 232 drives the zoom driving source 222 and the focusing driving source 223 so that the current absolute position of the variator lens in the direction of she optical axis and the calculated position at which the variator lens is to be set for which the positions are a result of the detection by the zoom encoder 225 are the same as the current absolute position of the focusing lens in the direction of the optical axis and the calculated position at which the focusing lens is to be set for which the positions are a result of the detection by the focus encoder 227.
In automatic focusing, the CPU 232 drives the focusing driving source 223 so that the output of the AF signal processing circuit 231 is at its peak.
Further, to obtain correct exposure, the CPU 232 controls the aperture diameter by driving the diaphragm driving source 224 so that the output of the diaphragm encoder 226 is a predetermined value which is the average value of the outputs of the Y signals having passed through the AE gate 229.
Next, an AF method using a TV signal will be described. Here, the above-described automatic focusing will be described in more detail. This method which uses the image pickup device of the imaging apparatus also as a sensor for performing automatic focusing is advantageous in cost because the number of parts is small compared to a case where a separate AF sensor is provided. Moreover, since the condition of the image on the imaging surface is directly detected, for example, even when lens barrel parts expand or contract due to a temperature change and this changes the focus position, the correct focus position can be detected according to the change.
FIG. 10 shows the principle of the TV-AF method. In the graph of FIG. 10, the horizontal axis shows the lens component position for focusing, and the vertical axis shows the high-frequency component (focus voltage) of the image sensing signal. In the figure, the peak of the focus voltage is reached at the position A shown by the arrow. The position A is the Lens position where the subject is in focus.
An example of a method for obtaining the focus voltage F will be described. FIG. 11(A) shows an actual image sensing field. Reference numeral 720 shows an angle of view. Reference numeral 718 shows an image signal extraction range for automatic focusing. Reference numeral 719 shows a subject image.
In FIG. 11(B), (a) shows the subject image within the image signal extraction range, and (b) shows an image signal (Y signal) of the subject image shown in (a).
Differentiating this signal, a waveform as shown in (c) is obtained, and converting it to an absolute value, (d) is obtained.
The signal (e) obtained by sampling and holding the signal (d) is the focus voltage E. This method uses the fact that, of the contrast signal of the subject image, a high-frequency component is highest when the subject is in focus. Various other methods are known as the method for producing the focus voltage.
Although a bypass filter for extracting only a high-frequency component is frequently used, it is also known to provide some kinds of properties of this filter, produce the focus voltage for a plurality of frequencies and ensure correct focus based on these pieces of information.
FIG. 12 shows the structure of a camera in which this automatic focusing apparatus is combined with an inner focusing lens.
At the imaging position designated by 805, an image pickup device such as a CCD is disposed. A luminance signal Y is produced through the image pickup device, and the information within the predetermined frame 718 (FIG. 11(A)) is taken into an AF circuit 821.
The AF circuit 821 obtains the focus voltage by the above-described method or the like, and determines whether the subject is in focus or out of focus, when the subject is out of focus, whether the blur is caused because the camera is focused on the background or on the foreground based on the obtained focus voltage, and the driving direction of a focusing lens 804B and the sign of the change of the focus voltage caused by the driving. Based on the result of the determination, the AF circuit 821 drives a focusing lens driving motor 822 in a predetermined direction.
According to a method as described above called TV signal automatic focusing, since the sensor which is an imager of the imaging apparatus is used also as the sensor for automatic focusing, the imaging condition of the imaging surface can be directly measured, so that the focus condition can be grasped with high accuracy.
Next, a zoom tracking method will be described. Although briefly touched on in the description of FIG. 9, when focusing is performed by a lens component situated at the rear of the variator, the path which the focusing lens should take during zooming differs according to the subject distance.
Therefore, by measuring both of the absolute positions, in the direction of the optical axis, of the variator lens and the focusing lens when zooming is started, clarifying based on this information the positional relationship which the two lenses take when zooming is performed and performing an operation such that the positions are maintained, focusing can be maintained during zooming. This operation is referred to as zoom tracking here.
As this method, Japanese Laid-Open No. H01-321416 shows a method such that focusing lens positions for a plurality of variator lens positions between the wide end and the tele end are stored for a plurality of subject distances, the locations at that time of the variator lens position and the focusing lens position on the map information stored in storage means or the like in a microcomputer are found when zooming is started, interpolation calculation is performed based on the data at the points, and the data stored closest to the side where the camera is focused on the foreground and the data stored closest to the side where the camera is focused on the background at the same focal length, and the focusing lens position at each focal length (variator position) is calculated.
FIG. 13 is a view explaining the tracking curve in the vicinity of the tele end. In this figure, the horizontal axis shows the variator lens position, and Vn shows the position of the tele end. The vertical axis shows the focusing lens position.
For example, it is assumed that P1, P4, P7 and P10 are stored for infinity and P2, P5, P8 and P11 are stored for 10 m. At this time, when zooming is performed from a condition of situating at the point P (condition where the subject distance is between 10 m and infinity at the tele end) in the direction toward the wide end, the positional relationship between the variator lens and the focusing lens is controlled so as to shift from P to PA, PB and PC in this order.
The positions of PA to PC are positions where the interpolation ratio between the stored upper and lower tracking curves LL2 and LL1 is fixed.
Next, an interchangeable lens system will be described. Conventionally, interchangeable lens systems in which shooting lenses are interchangeable for imaging apparatuses have been frequently used.
FIG. 14 shows an example of a shooting system using an interchangeable lens. As this interchangeable lens 900, like the above-described one, a four-component zoom lens system is used comprising from the subject side a positive lens component, a negative lens component, a positive lens component and a positive lens component. However, a lens system of a different structure may be used.
Reference numeral 911 shows a stationary front lens. Reference numeral 912 shows a variator lens performing zooming by moving in the direction of the optical axis. Reference numeral 936 shows a stop. Reference numeral 913 shows a stationary afocal lens. Reference numeral 914 shows a focusing lens. The focusing lens 914 functions as a compensator during zooming as well as performing focusing when the subject distance is changed.
Reference numerals 945, 952 and 937 shows the driving sources for the variator, the stop and the focusing lens, respectively. The driving sources 945, 952 and 937 are driven by a lens microcomputer 910 through driving circuits 961, 951 and 962, respectively.
On the side of a camera 1000, three image pickup devices 1003 through 1005 such as CCDs are provided. The signals output from the image pickup devices 1003 to 1005 are amplified by amplifiers 1015 through 1017, respectively. These signals are input to a signal processing circuit 1152 where an image signal of a predetermined level is produced. The produced image signal is transmitted to a camera microcomputer 1009.
The two microcomputers 910 and 1009 are coupled by a communications path connected through contacts 918 and 1007. By this, various kinds of signals are exchanged.
For example, when the focus voltage for the above-described TV signal automatic focusing is produced in the camera signal processing circuit 1152 on the side of the camera 1000, the information is transmitted from the camera microcomputer 1009 to the lens microcomputer 910.
The lens microcomputer 910 determines whether the subject is in focus or out of focus (the direction and the degree of blur) based on the signal information, decides in which direction and how fast the focusing lens 914 is driven, and drives the focusing driving source 937 through the driving circuit 962.
Next, the image pickup device will be described. As CCD image pickup devices for consumer video cameras, ones with diagonal sizes of approximately 6 mm and 4 mm called a ⅓-inch type and a ¼-inch type, respectively, are in the mainstream. In these sizes, for example, 310,00 pixels are provided.
For digital still cameras, a CCD of an approximately ½-inch type (with a diagonal size of 8 mm) having two million pixels is also used.
In the case of general small print sizes, digital cameras using a CCD with such a large number of pixels are reaching the ability to ensure image quality bearing comparison with that of photos taken by conventional film cameras when conditions are met.
In such video cameras, the permissible confusion circle diameter is approximately 12 to 15 μm, and in digital still cameras, the permissible confusion circle diameter is approximately 7 to 8 μm. These figures are far smaller than the permissible confusion circle diameters 33 to 35 μm of the conventional 135 film format.
This is because the diagonal size of the image plane is far smaller than 43 mm of the 135 film format as mentioned above. Moreover, it is conceived that these figures are smaller when the pixel size of the CCD is smaller.
From a different point of view, in imaging apparatuses using a CCD, the focal length for obtaining the same angle of view is smaller than that of 135 film cameras because the image size is smaller.
For example, the angle of view obtained at a standard focal length of 40 mm in 135 film cameras is obtained at a standard focal length of 4 mm in imaging apparatuses using a ¼-inch CCD. Therefore, the depth of field obtained when shooting is performed at the same F-number (the aperture value of the stop unit) in imaging apparatuses using the CCD is extremely large compared to that obtained in film cameras.
Since the depth of field is obtained by the permissible confusion circle diameter×the F-number (aperture value) as well known, for example, in the case of F2, the depth of field (one side) of 135 film cameras is 0.035×2=0.07 mm, whereas the depth of field of ½-inch-type imaging apparatuses is 0.007×2=0.014 mm, which is smaller than the depth of field of 135 film cameras.
As CCDs having the same diagonal size as mentioned above, for example, ⅓-inch-type CCDs of 6 mm, ones of various specifications are known such as ones intended for increasing resolution by increasing the number of pixels from one million to two million and further to three million in the future, and ones regarding dynamic range and sensitivity important without excessively reducing the pixel size.
Next, a light quantity adjusting method will be described. In imaging apparatuses using an image pickup device such as a CCD as the image sensor like video cameras and digital still cameras, it is common practice to automatically obtain optimum exposure by controlling the aperture diameter with the stop so that the level of the luminance signal of the CCD is in a predetermined range.
As the stop, one using two stop blades and having a rhombic aperture and an iris stop using five or six stop blades are known.
When the aperture diameter of the stop is reduced, a problem arises in that image quality is degraded by diffraction. Therefore, in these imaging apparatuses, the control range of the aperture diameter of the stop is generally limited to a range where no image degradation occurs or image degradation is not a significant problem even if it occurs.
This is performed by the microcomputer grasping the current aperture value and not using the F-numbers on the small aperture side of a predetermined F-number.
However, when the usable aperture range is limited like this, it is difficult to adjust the light quantity so as to be optimum only by the stop for a wide range of brightness of the actual field.
Therefore, the brightness range adjustable by the same aperture control (for example, minimum to F8) is increased by integrally attaching an ND filter to the stop blade so that the ND filter covers the aperture when the aperture diameter decreases. There are cases where a method changing the charge accumulation time of the CCD (shutter speed) is combined.
Examples of ND filters include not only the above-described one integrally attached to the stop blade and driven but also one having a driving source provided specifically therefor and whose amount of insertion into the optical path is controlled separately from the stop.
Next, the shooting lens will be described. The shooting lens is designed and manufactured so that necessary resolution performance, or MTF (modulation transfer function) performance, determined by the pixel pitch of the CCD used is obtained.
Moreover, the shooting lens has an effective image circle determined by the size of the CCD.
In the imaging apparatus structured as described above, many functions are based on the CCD specifications and designed so as to be optimized for the CCD specifications.
First, with respect to AF, since the focal point is determined based on the peak of the high-frequency component of the image signal obtained from the CCD, the movement amount of one step when the focusing lens is driven by a stepping motor is set based on the permissible confusion circle determined by the pixel Ditch of the CCD and the minimum F-number of the stop.
When the direction of the best focus is searched for by so-called wobbling (minute reciprocating driving in the direction of the optical axis) of the lens, the wobbling amount corresponding to the F-number is also determined by the permissible confusion circle specification (and by extension, to the CCD specification), and the level when it is determined whether the subject is in focus or out of focus is also determined in association with the CCD.
With respect to automatic exposure control AE, the F-number at which image degradation due to small aperture diffraction occurs is determined by the pixel pitch of the CCD. Exposure is controlled so that the F-numbers on the small aperture side of this F-number are not used.
With respect to the effective image circle, in designing and manufacturing the lens, the lens is designed in accordance with the size of the CCD so that no eclipse occurs.
With respect to the resolution performance, the design value is determined by the pixel pitch specifications of the CCD and the like in designing and manufacturing the lens.
As described above, lens interchangeable imaging apparatuses are designed so that excellent imaging performance is obtained for all the interchangeable lenses according to the specifications of the CCD used by the imaging apparatus.
However, the CCD corresponds to the film in film cameras, and characteristics (for example, the number of pixels, the sensitivity and the dynamic range) differ according to the specifications even though the image size is the same, such that high image quality is required although the sensitivity is low as mentioned above or that high sensitivity is required, in accordance with the object of the shooting.
Moreover, CCDs are decreasing in pixel size year after year as semiconductor manufacturing technology improves, and specifications are changing year after year to extend the range of choices.
Even if an interchangeable lens shooting system is designed with one kind of CCD in mind under such circumstances, the entire system will soon be rendered obsolete as the CCD improves, or every time a new CCD is created, it is necessary to re-design the lens in accordance with the latest CCD.
Moreover, when a lens always satisfying the highest performance of the CCD is prepared, since it is necessary that sufficient MTF be obtained even when the system is designed with a CCD having the highest pixel size in mind, even users not requiring such high image quality are obliged to use a lens of unnecessarily high performance (in many cases, the lens size increases as the MTF increases).